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Suspended sediment concentration (SSC) is one of the fundamental topics in sediment study. The parameterization of the SSC profile of silty sediments is still under-researched. This study focuses on the mean SSC profile for silty sediments under non-breaking wave-dominant conditions. First, inspired by a 1DV model, different types of the distribution of mean eddy viscosity were proposed, i.e., a toe-type distribution over flat bed and a constant-toe type distribution over rippled bed. Then, the time-averaged diffusion equation for suspended sediment transport was analytically solved, and expressions for the mean SSC profiles were derived. The expressions involve several basic physical processes, including the effects of bed forms, stratification, hindered settling and mobile bed. Verification using a number of experimental datasets showed that the proposed expressions can properly calculate the mean SSC for silt and are applicable for sand as well. In conclusion, this research provides an approach to estimate the mean SSC for silty sediments under wave-dominant conditions, which is expected to be applicable for engineering practice and numerical modelling. • Formulations for mean SSC (suspended sediment concentration) profile of silty sediments were derived. • Several essential physical processes were considered in the formulations. • The SSC profiles over rippled bed and hydrodynamically plane bed were simulated. [ABSTRACT FROM AUTHOR]

Sediment incipient motion is a fundamental issue in sediment transport theory and engineering practice. Silt has transitional behavior between cohesive and non-cohesive sediment and its incipient motion is still poorly understood. This study aims to find an expression for incipient motion from silt to sand from a unified perspective and analysis. From the analysis of forces, using the derivation method for the Shields curve, an expression for sediment incipient motion is proposed for both silt and sand under conditions of combined waves and currents. The differences and similarities in the sediment motion threshold were analyzed under the effects of waves and currents, and fine and coarse sediment. The Shields number was revised by introducing the cohesive force and additional static water pressure, which indicates that this study could be seen as an extension of the Shields curve method for silt. A number of experimental datasets as well as field data were used to verify the formula. The effect of bulk density on fine sediment was discussed and tested using experimental data. [ABSTRACT FROM AUTHOR]

Peak river flows transport fine sediment, nutrients, and contaminants that may deposit in the estuary. This study explores the importance of peak river flows on sediment dynamics with special emphasis on channel network configurations. The Sacramento-San Joaquin Delta, which is connected to San Francisco Bay (California, USA), motivates this study and is used as a validation case. Besides data analysis of observations, we applied a calibrated process-based model (D-Flow FM) to explore and analyze high-resolution (∼100 m, ∼1 h) dynamics. Peak river flows supply the vast majority of sediment into the system. Data analysis of six peak flows (between 2012 and 2014) shows that on average, 40 % of the input sediment in the system is trapped and that trapping efficiency depends on timing and magnitude of river flows. The model has 90 % accuracy reproducing these trapping efficiencies. Modeled deposition patterns develop as the result of peak river flows after which, during low river flow conditions, tidal currents are not able to significantly redistribute deposited sediment. Deposition is quite local and mainly takes place at a deep junction. Tidal movement is important for sediment resuspension, but river induced, tide residual currents are responsible for redistributing the sediment towards the river banks and to the bay. We applied the same forcing for four different channel configurations ranging from a full delta network to a schematization of the main river. A higher degree of network schematization leads to higher peak-sediment export downstream to the bay. However, the area of sedimentation is similar for all the configurations because it is mostly driven by geometry and bathymetry. [ABSTRACT FROM AUTHOR]

Abstract: Observations of storm-induced fluid mud dynamics have been conducted at the North Passage deepwater navigational channel (DNC) of the Yangtze Estuary in October to December 2010, during the occurrence of a cold-air front. The measurement data reveal that just after the critical wind wave event, a large amount of fine sediment was trapped in a state of fluid mud along the channel. The observed thickness of the fluid mud was up to about 1–5m, which caused some significant economic and safety problems for shipping traffic in the Yangtze Delta area. The mechanisms and transport processes of the storm-induced fluid mud are analyzed and presented from the angles of both process-oriented and engineering-oriented methods. With the help of hydrodynamics and wave modeling, it could be inferred that the behavior of the storm-induced fluid mud event mainly depends on the overall hydrodynamic regimes and the exchanges of sediment, which is released by storm-wave agitation from adjacent tidal flats. These sediments are accumulated as fluid mud, and subsequently oscillate and persist at those locations with weaker longitudinal residuals in the river- and tide-dominated estuary. In addition, the downslope transport of fluid mud is also thought to have stimulated and worsened the fluid mud event observed in this study. Our modeling results and observations demonstrate that: (1) the transport of fluid mud is an advective phenomenon determining the central position of fluid mud layer along the channel, and it's also a tidal energy influenced phenomenon controlling the erosion and accumulation of fluid mud; and (2) both suspended particulate matter availability and local residual flow regime are of critical importance in determining the trapping probability of sediment and the occurrence of fluid mud. [Copyright &y& Elsevier]

Ab Razak, M.S., Dastgheib, A., Suryadi, F.X., Roelvink, D. 2014. Headland structural impacts on surf zone current circulation, Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 065-071, ISSN 0749-0208. The paper presents the morphodynamic investigation of embayed beaches through the impact of structural headlands. An XBeach model was successfully applied to predict the surf zone current pattern and to predict morphological features of three different embayment scales for low-moderate-high wave energy events. The formation of central rip currents in embayed beaches is linked to the presence of a sand bar, while topographical headland rips developed adjacent to the headland boundary are caused by the geological control of the headland structure itself. The effect of moderate and high waves has resulted to a decreased number of central rips in a longer embayment thus limiting the beach circulation to the cellular type. This type of rips could provide an initial insight into the offshore transport of sand from the coast. Whether wave breaking occurs outside or inside the embayment determines the initiation of large scale rip currents, megarips. Several factors that lead to the generation of megarip currents in literature should be taken into account as a guideline for morphodynamic studies in a high energy embayed coast. Nevertheless, the characteristics of surf zone current circulation in all cases presented in this study complies with the description of the theoretical embayment scaling parameter (δ') (Short and Masselink, 1999 and Castelle and Coco, 2012). Further establishment of this parameter is needed to properly describe the surf zone current circulation in embayed beach systems. [ABSTRACT FROM AUTHOR]

pplications of process-based morphodynamic models are often constrained by limited availability of data on bed composition, which may have a considerable impact on the modeled morphodynamic development. One may even distinguish a period of 'morphodynamic spin-up' in which the model generates the bed level according to some ill-defined initial bed composition rather than describing the realistic behavior of the system. The present paper proposes a methodology to generate bed composition of multiple sand and/or mud fractions that can act as the initial condition for the process-based numerical model Delft3D. The bed composition generation (BCG) run does not include bed level changes, but does permit the redistribution of multiple sediment fractions over the modeled domain. The model applies the concept of an active layer that may differ in sediment composition above an underlayer with fixed composition. In the case of a BCG run, the bed level is kept constant, whereas the bed composition can change. The approach is applied to San Pablo Bay in California, USA. Model results show that the BCG run reallocates sand and mud fractions over the model domain. Initially, a major sediment reallocation takes place, but development rates decrease in the longer term. Runs that take the outcome of a BCG run as a starting point lead to more gradual morphodynamic development. Sensitivity analysis shows the impact of variations in the morphological factor, the active layer thickness, and wind waves. An important but difficult to characterize criterion for a successful application of a BCG run is that it should not lead to a bed composition that fixes the bed so that it dominates the 'natural' morphodynamic development of the system. Future research will focus on a decadal morphodynamic hindcast and comparison with measured bathymetries in San Pablo Bay so that the proposed methodology can be tested and optimized. [ABSTRACT FROM AUTHOR]

Fluvial, tidal, and combined hydro-morphodynamics interaction in a complex, seasonal, sediment transport regime has been the subject of extensive research. It becomes particularly challenging when there is limited data. The Ganges-Brahmaputra-Meghna (GBM) Delta is one example of a huge system lacking data. Bathymetric data simultaneously covering the rivers and estuaries is hardly present, let alone sequences of bathymetries or a system-wide sediment budget. Hence it is difficult to understand and predict future developments. This research aims to make a sediment budget for the GBM delta with a process-based model. It is a first-ever sediment budget simulation for the GBM system. A process-based morphological model, Delft3D, has been used to reproduce the bathymetric evolution over time and the associated sediment budget. This chapter demonstrates the possibilities for the application of a robust modeling system to assess the morphodynamic evolution and sediment budget and pathways. The Ganges and Jamuna rivers carry sediment load in the ranges of 216–1038 million tonnes/yr and 80–228 million tonnes/yr respectively. The total accumulation in the estuary system is 1150 million tonnes/yr, out of which more than eighty percent of sediment is in suspension. The model results show that about 22% of the total sediment coming into the system is deposited in the floodplains and tidal plains and causes river morphology changes. The rest of the sediment is lost to the pro-delta, to the deep ocean bed, or leaves the domain. The results also indicate that Padma, Gorai, Pussur-Sibsa, Bishkhali, Shahbajpur channel, Lower Meghna, Tentulia Channel, and Arial Khan rivers are mainly in the aggrading phase, whereas, the Ganges, Jamuna, and Baleshwar are in the degrading phase. This particular delta model offers many opportunities to compare with sediment data, where it deals with a poorly surveyed area. • Process-response relation can be checked with Delft-3D.Sediment budget in the GBM or Bengal delta. • Riverbed aggradation and degradation is a sign of dynamic system. • Delta model offers many opportunities. [ABSTRACT FROM AUTHOR]

Rivers supply sediment to estuaries and alter estuarine tidal wave regimes. However, the long-term (> century) impact of seasonally varying river discharges on estuarine morphodynamics is not well understood. Process-based models are useful tools to analyze these morphodynamic processes. This study aims to explore modeling techniques to efficiently include the impact of discharge seasonality on long-term estuarine morphodynamic development by a process-based morphodynamic model. Inspired by the dimensions and hydraulic forcing of the Yangtze River estuary, we evaluate three methodologies to schematize a river discharge hydrograph in a 1-D morphodynamic model. These are a representative constant discharge, a non-compressed hydrograph and a compressed hydrograph. To shorten computational time, the latter methodology decreases the timescale of seasonal river flow variations compared to the tidal timescale. Sensitivity analysis suggests that the yearly averaged sediment transport controls long-term morphodynamic development. Morphodynamic equilibrium is reached after millennia when deposition during a high river flow period is balanced by erosion during a low river flow period. An important finding is that a yearly hydrograph can be squeezed in time up to a factor of 20, without leading to unrealistic river–tide interactions. Applying a constant river discharge may not produce equilibrium bed profiles that are similar to that under hydrograph forcing due to the non-linearity in sediment transport. The long characteristic timescales in defining morphodynamic equilibrium indicate that a long-term view is needed to interpret large scale estuarine morphodynamics. Though simple and highly schematized, the modeling experiments in this study provide conceptual understanding of morphodynamics in river-influenced estuaries. [ABSTRACT FROM AUTHOR]

The estuaries and coastal zones of the Vietnamese Mekong Delta located at the end of the Mekong Basin are susceptible to a large variety of natural threats and human interventions such as upstream dam construction, climate change and sea level rise. In this study, we investigated sediment dynamics and morphodynamic changes in the Mekong estuaries and coastal zone, using a well-calibrated Delft3D-4 model for a 10-years period simulation. For the first time, the impacts of different drivers were distinguished to guide future adaptation strategies and policies. Our investigations pointed out that the influences of upstream sediment reduction and large-scale sand extraction would cause substantial modifications in the subaqueous delta region. Besides human-induced impacts, modeling results also showed that the sediment volume and spatial distribution changed through the simulated period according to monsoonal variation. The influence of each driver, however, not only varied in space but also time. In the flood season, the Mekong and Bassac Rivers provided a large amount of sediment (more than 90%) that was deposited on the delta front due to coastal processes. The sediment quantity transported along the coastline changed with the monsoon and was in a dominantly south-west direction due to the northeast monsoon, especially in November, December and January. As a natural process, in the estuarine region, erosion and deposition occurred alternately, but the south-west coast region (belonging to the Ca Mau peninsula) was dominated by erosion. This is because of the formation of sand bars in front of the Bassac estuaries, which influences the capacity of sediment transportation to the south-west. Consequently, navigation capacity will reduce significantly in the estuaries and coastline erosion in the Ca Mau peninsula leads to substantial loss of houses and assets of local people. Urgent actions such as stopping sand mining or modifying dam designs are needed to sustain the delta coastline. • A well-calibrated Delft3D model against observed and remote sensing data. • Sediment transport is strongly driven by both river discharges and monsoons. • Long-term sand bar formation diverted the direction of current sediment flows. • Human activities will lead to the severe recession of the delta. [ABSTRACT FROM AUTHOR]

Abstract This paper aims to fundamentally assess the resilience of salt marsh-mudflat systems under sea level rise. We applied an open-source schematized 2D area model (Delft3D) that couples intertidal flow, wave-action, sediment transport, geomorphological development with a population dynamics approach including temporal and spatial growth of vegetation and bio-accumulation. Wave-action maintains a high sediment concentration on the mudflat while the tidal motion transports the sediments within the vegetated marsh areas during flood. The marsh-mudflat system attained dynamic equilibrium within 120 years. Sediment deposition and bio-accumulation within the marsh make the system initially resilient to sea level rise scenarios. However, after 50–60 years the marsh system starts to drown with vegetated-levees being the last surviving features. Biomass accumulation and sediment supply are critical determinants for the marsh drowning rate and survival. Our model methodology can be applied to assess the resilience of vegetated coast lines and combined engineering solutions for long-term sustainability. Graphical abstract Image 1 Highlights • Realistic marsh-mudflat system in near equilibrium within a century (Video 2.6). • Wave action is the primary trigger for sediment supply to the mudflat. • The marsh-mudflat system drowns under sea level rise scenarios. • Drowning depends on spatially and temporally varying morphology. [ABSTRACT FROM AUTHOR]

Abstract Experiments and field observations have revealed that when silt and very fine sand are subject to oscillatory wave motion, a high shear flow layer and a high concentration layer (HCL) exist near the bottom. The behavior of the HCL is still under researched. Firstly, an intra-wave process based 1DV model was established for fine sediment transport under the combined action of waves and currents. Some key processes that were included in the model are represented through approaches for different bed forms (rippled bed and 'flat bed'), hindered settling, stratification, reference concentration and critical shear stress. A number of experimental datasets were collected to verify the model, which shows that the model is able to properly simulate the flow and sediment dynamics. Secondly, sensitivity analyses were carried out on some factors which would impact the suspended sediment concentration (SSC) profile of the HCL by the 1DV model, such as bed forms, flow dynamics, stratification effects, mobile bed effects and hindered settling. Results show that bed forms play a significant role in the HCL and determination of the shape of the concentration profile. When a current is imposed, the SSC profiles become smoother; however, sediment concentration in the lower HCL is still dominated by the wave motions. For finer sediment, the stratification effects and the mobile bed effects strongly impact the HCL. In conclusion, this paper provides a tool for the study of the HCL and an evaluation of several impact factors on the HCL. Highlights • An intra-wave process based 1DV model was established for silty sediment movement under the combined action of waves and currents. • Important factors impacting the high concentration layer (HCL) are bed forms, flow dynamics, stratification and mobile bed effects. • Bed forms play a significant role in the HCL and determination of the shape of the concentration profile. • For fine sediment, the stratification effects and the mobile bed effects strongly impact the HCL. [ABSTRACT FROM AUTHOR]

Fluvial sediment is the major source for the formation and development of the Mekong Delta. This paper aims to analyse the dynamics of suspended sediment and to investigate the roles of different processes in order to explore flux pattern changes. We applied modelling on two scales, comprising a large-scale model (the whole delta) to consider the upstream characteristics, particularly the Tonle Sap Lake's flood regulation, and a smaller-scale model (tidal rivers and shelf) to understand the sediment processes on the subaqueous delta. A comprehensive comparison to in-situ measurements and remote sensing data demonstrated that the model is capable of qualitatively simulating sediment dynamics on the subaqueous delta. It estimates that the Mekong River supplied an amount of 41.5 mil tons from April 2014 to April 2015. A substantial amount of sediment delivered by the Mekong River is deposited in front of the river mouths in the high flow season and resuspended in the low flow season. A sensitivity analysis shows that waves, baroclinic effects and bed composition strongly influence suspended sediment distribution and transport on the shelf. Waves in particular play an essential role in sediment resuspension. The development of this model is an important step towards an operational model for scientific and engineering applications, since the model is capable of predicting tidal propagation and discharge distribution through the main branches, and in predicting the seasonal SSC and erosion/deposition patterns on the shelf, while it is forced by readily available inputs: discharge at Kratie (Cambodia), GFS winds, ERA40 reanalysis waves, and TPXO 8v1 HR tidal forcing. [ABSTRACT FROM AUTHOR]

Bar-built or barrier estuaries (here referred to as Small tidal inlets, or STIs), which are commonly found along wave-dominated, microtidal mainland coasts, are highly likely to be affected by climate change (CC). Due to their pre-dominance in tropical and sub-tropical regions of the world, many STIs are located in developing countries, where STI related activities contribute significantly to the national GDPs while community resilience to coastal changes is low, with the corollary that CC impacts on STIs may lead to very serious socio-economic consequences. While assessing CC impacts on tidal inlets is in general difficult due to inherent limitations of contemporary numerical models where long term morphodynamic simulations are concerned, these difficulties are further exacerbated due to the lack of sufficient model input/verification data in often data poor developing country STI environs. As a solution to this problem, Duong et al. (2016) proposed two different process based snap-shot modelling approaches for data poor and data rich environments. This article demonstrates the application of Duong et al.’s (2016) snap-shot modelling approach for data poor environments to 3 case study sites representing the 3 main STI types; Permanently open, locationally stable inlets (Type 1), Permanently open, alongshore migrating inlets (Type 2) and Seasonally/Intermittently open, locationally stable inlets (Type 3). Results show that Type 1 and Type 3 inlets will not change Type even under the most extreme CC driven variations in system forcing considered here. Type 2 inlets may change into Type 1 when CC results in a reduction in annual longshore sediment transport. Apart from Type changes, CC will affect the level of inlet stability and some key behavioural characteristics (e.g. inlet migration distances, inlet closure times). In general, CC driven variations in annual longshore sediment transport rates appear to be more relevant for future changes in inlet stability and behaviour, rather than sea level rise as commonly believed. Based on model results, an inlet classification scheme which, for the first time, links inlet Type with the Bruun inlet stability criteria is presented. [ABSTRACT FROM AUTHOR]

A laboratory experiment was conducted to investigate the dynamics of cross-shore sediment transport across a fringing coral reef. The aim was to quantify how a highly bimodal spectrum of high-frequency (sea-swell) and low-frequency (infragravity and seiching) waves that is typically present on coral reef flats, influences the various sediment transport mechanisms. The experiments were conducted in a 55 m wave flume, using a 1:15 scale fringing reef model that had a 1:5 forereef slope, a 14 m long reef flat, and a 1:12 sloping beach. The initial 7 m of reef flat had a fixed bed, whereas the back 7 m of the reef and the beach had a moveable sandy bed. Four seven-hour irregular wave cases were conducted both with and without bottom roughness elements (schematically representing bottom friction by coral roughness), as well as for both low and high still water levels. We observed that the wave energy on the reef flat was partitioned between two primary frequency bands (high and low), and the proportion of energy within each band varied substantially across the reef flat, with the low-frequency waves becoming increasingly important near the shore. The offshore transport of suspended sediment by the Eulerian mean flow was the dominant transport mechanism near the reef crest, but a wide region of onshore transport prevailed on the reef flat where low-frequency waves were very important to the overall transport. Ripples developed over the movable bed and their properties were consistent with the local high-frequency wave orbital excursion lengths despite substantial low-frequency wave motions also present on the reef flat. This study demonstrated that while a proportion of the sediment was transported by bedload and mean flow, the greatest contributions to cross-shore transport were due to the skewness and asymmetry of the high and low-frequency waves. [ABSTRACT FROM AUTHOR]

On October 29, 2012, storm surge and large waves produced by Hurricane Sandy resulted in the formation of a breach in eastern Fire Island, NY. The goals of this study are to gain a better understanding of the physical processes that govern breach behavior and to assess whether process-based models can be used to forecast the evolution of future breaches. The wilderness breach grew rapidly in size during the first winter following formation. Growth of the breach was accompanied by the formation of a complex of flood shoals inside Great South Bay, a primary channel that flowed through the eastern part of the flood shoals, and an ebb shoal on the ocean side of the breach. From the summer of 2013 through late 2015, the breach continued to change and evolve, albeit at a much slower pace than in the first year after formation. A hybrid combination of Delft3D and XBeach models is used to hindcast the morphodynamic evolution of the wilderness breach over the first three years after formation. The formation of the breach during Hurricane Sandy is not part of the simulations. Model simulations are initiated with a post-storm topography in which the breach is already present. The models are capable of hindcasting the main morphodynamic changes of the wilderness breach. The spatial patterns, as well as the bulk statistics, such as breach geometry and sediment volume changes, are reasonably well reproduced by the model. The model sheds light on previously unknown processes of breach evolution, especially regarding sediment transport and flow regimes within the breach complex. • A hybrid combination of Delft3D and XBeach models hindcasts the morphodynamic evolution of a barrier island breach. • Model results are validated against a wide range of field observations. • The models shed light on previously unknown processes of breach evolution regarding sediment transport and flow regimes. [ABSTRACT FROM AUTHOR]

• Morphodynamic hindcast of evolution of multiple-inlet system on barrier-lagoon coast was studied by a process-based model. • The evolution mechanism of two deep channels were studied respectively under a series of initial conditions. • Numerical results revealed how the shape of Barrier Islands and changes of sediment supply affect the morphologic evolution. Understanding the long-term morphodynamic evolution of tidal inlet systems and their control mechanisms has great significance in the management and sustainable development of coastal regions. This study addresses this issue by applying a process-based morphological model (Delft3D) to hindcast morphodynamic evolution of the Laolonggou tidal inlet (LTI) for a duration of 68 years and then analyze the mechanism of morphologic evolution and factors affecting it in tidal inlets. The morphodynamic model is validated against two periods, a natural stage (1938–1983), and a recent erosion stage with reservoir construction (1983–2006). Results show that the evolution pattern, sediment volume changes, and hypsometry curves simulated by the model agree well with field data. A series of scenarios were designed to analyze the mechanism of morphologic evolution and impacting factors. The model results reveal that the asymmetry of the tidal currents in the channels leads to tide residual sediment transport, which causes morphodynamic evolution. The inconsistency of tidal flow path, i.e., ebb dominance occurs in the west channel and flood dominance occurs in the east channel, is an important mechanism for the evolution of the whole system. In addition, the decrease of sediment supply caused by reservoirs in Luanhe River, and extension of the barrier island contribute to the evolution of LTI. Although this study is site-specific, the results are expected to provide valuable information for sustainable management of tidal inlet systems. [ABSTRACT FROM AUTHOR]